FIG. 6 shows a conventional reference voltage circuit 100. The reference voltage circuit 100 is a circuit for converting a DC power supply voltage VDD to a stable reference voltage VREF, and particularly it is designed so as to supply a reference voltage VREF which is adjusted to a fixed value against variation of background temperature. The conventional reference voltage circuit 100 is equipped with an operational amplifier OP, a first resistor R1, a second resistor R2, a third resistor R3, a first diode D1 and a second diode D2.
The second diode D2 is a diode group containing plural diodes connected to one another in parallel, and each diode has the same specification as the first diode D1.
Positive and negative power supply lines 36 and 37 are connected to the positive and negative terminals of a DC power source, and the positive and negative power supply lines 36 and 37 are connected to the positive and negative power supply terminals of the operational amplifier OP. One end of the first resistor R1 is connected to the output terminal of the operational amplifier OP, and the other end is connected to the non-inverting input terminal of the operational amplifier OP. One end of the second resistor R2 is connected to the output terminal of the operational amplifier OP, and the other end thereof is connected to the inverting input terminal of the operational amplifier OP. One end of the third resistor R3 is connected to the inverting input terminal of the operational amplifier OP, and the other end thereof is connected to the anode terminal of the second diode D2. The cathode terminal of the second diode D2 is connected to the negative power supply line 37.
The second diode D2 is inserted in the forward direction with respect to the negative power supply line 37. The anode terminal of the first diode D1 is connected to the non-inverting input terminal of the operational amplifier OP, and the cathode terminal thereof is connected to the negative power supply line 37. The first diode D1 is inserted in the forward direction with respect to the negative power supply line 37. JP-A-2003-7837 discloses an example of this type of reference voltage circuit.
When the forward voltage drop VD1[T] of the first diode D1 is represented by an equation, the following equation (1) is achieved.
                                          V                          D              ⁢                                                          ⁢              1                                ⁡                      [            T            ]                          =                              V            BG                    -                                    (                                                V                  BG                                -                                                      V                                          D                      ⁢                                                                                          ⁢                      1                                                        ⁡                                      [                                          T                      0                                        ]                                                              )                        ⁢                          T                              T                0                                              -                                    (                              η                -                1                            )                        ⁢                          kT              q                        ⁢            ln            ⁢                                                  ⁢                          T                              T                0                                                                        (        1        )            
T represents the temperature achieved by representing the background temperature of the reference voltage circuit 100 as the absolute temperature. T0 represents a reference absolute temperature, and it may be set to 20° C. (represented by Celsius), for example). VBG represents the band gap voltage of a pn junction contained in the first diode D1, and it is an inherent value in material. η represents a constant dependent on the manufacturing process of the reference voltage circuit 100, and it is normally equal to about 4. k represents the Boltzmann constant, and q represents the quantity of electric charge of one electron. The equation (1) is used in the other embodiments, and the symbols of the equation (1) has the same meaning as described above.
As well known, the reference voltage VREF[T, VDD] output from the reference voltage circuit 100 varies while following the background temperature T and the DC power supply voltage VDD. The variation of the reference voltage with respect to the background temperature can be represented by the following equation (2). Symbols achieved by adding numerals to symbols R representing the resistors represent the resistance values of the resistors added with the numbers.
                                          V            REF                    ⁡                      [            T            ]                          =                                            V                              D                ⁢                                                                  ⁢                1                                      ⁡                          [              T              ]                                +                                                    R                2                                            R                3                                      ⁢                          k              q                        ⁢            ln            ⁢                                                  ⁢                          (                                                nR                  2                                                  R                  1                                            )                        ×            T                                              (        2        )            
n represents the number of diodes constituting the second diode D2. Or, n also represents the ratio between the area constituting the pn junction of the first diode D1 and the area constituting the pn junction of the second diode D2.
In the conventional reference voltage circuit 100, when the equation (1) is substituted into the equation (2), the resistance values of the respective fixed resistors R1, R2, R3 are adjusted so that the primary term of the absolute temperature T of the equation (1) and the primary term of the absolute temperature T of the equation (2) are offset with each other, whereby the effect of the variation of the background temperature T on the reference voltage VREF is suppressed.
However, as shown in the equation (1), higher order terms concerning the background temperature exist actually. Accordingly, when a more stable reference voltage VREF is needed, the effect of the higher order terms must be considered. The higher order terms concerned cannot be offset by merely adjusting the resistance values of the respective fixed resistors R1, R2, R3.
Furthermore, it is known that the reference voltage VREF[T, VDD] of the conventional reference voltage circuit 100 is apt to vary while following variation of the DC power source voltage VDD. This phenomenon is caused by the fact that the offset voltage of the operational amplifier OP varies while following the variation of the DC power source voltage VDD. For example, when a battery or the like is used as the DC power source, the above phenomenon appears because the DC power source voltage greatly varied with time lapse.